The present invention relates to methods and apparatus for determining the characteristics of material surrounding an open hole well bore and in particular, relates to methods and apparatus for radio frequency resistivity well logging in which the true formation resistivity, flushed zone resistivity and invasion diameter are uniquely determinable by means of in situ measurements taken in a well borehole.
It has been conventional practice to log the electrical characteristics of earth formations in the vicinity of a well borehole in order to determine location of oil-bearing strata. There are only three material parameters which affect an electromagnetic wave, whether the wave gets from one point to another by induction or by propagation. They are conductivity (resistivity), magnetic susceptibility and dielectric constant. Conductivity provides an indication of the energy absorbing characteristics of the medium, while magnetic susceptibility and dielectric constant give a measure of the energy storing capacity of a material. The magnetic susceptibility of most earth materials has relatively little effect on electromagnetic waves and is of very little use in electrical logging techniques. Dielectric constant has considerable variation in the earth and has a large influence on high-frequency propagation but very little influence at low frequencies. It is well known that conductivity or resistivity has wide variation in value for earth materials and strongly affects all electromagnetic waves.
A propagating electromagnetic wave has two fundamental characteristics, amplitude and phase. By comparing the amplitude and phase of an electromagnetic wave as it passes receivers, propagation characteristics due to the formation may be studied. Measurement of these two characteristics in terms of wave travel time and attenuation may be used to determine the dielectric constant and/or the resistivity of the media through which the wave is propagated.
Study has indicated that four general frequency ranges exist which allow measurement of the formation effects. These four frequency regions are induction, low radio frequency propagation, high radio frequency propagation, and ultra high radio frequency propagation. The relative influence of resistivity (R.sub.t) and dielectric constant (.epsilon..sub.t) in these four ranges is shown in the following table (where R.sub.t = 20.OMEGA.m and .epsilon..sub.t = 20):
RELATIVE INFLUENCE ______________________________________ APPROX. RESIS- DIELE- WAVELENGTH IN TYPE OF LOG TIVITY CTRIC EARTH FORMATION ______________________________________ Induction 3.4 k Meter (20 to 50 kHz) 1.0 0.0004 (11,000 ft.) Low Radio Frequency Propagation 1.0 0.004 34 meter Resistivity (110 ft.) (2 MHz) High Radio Frequency Dielectric and 1.0 0.7 2.2 meter Resistivity (7.2 ft.) (30 MHz) Ultra-High Radio Frequency Dielectric 1.0 22.0 .061 meter (1 to 3 GHz) (0.2 ft.) ______________________________________
From the table, it can be seen that resistivity is the only parameter that materially influences the measurements in the induction and low radio frequency propagation range. The resistivity and dielectric constant have about the same magnitude of influence in the high radio frequency propagation region. In the ultra high radio frequency region, the dielectric constant dominates the wave travel time while the resistivity influences wave attenuation.
Electrical induction logging has been practiced for many years. In conventional induction logging, a well logging sonde is provided having a transmitter coil (or array of coils) and a receiver coil (or array of coils) at longitudinally spaced intervals from the receiver coil. Usually, an alternating current, in a range of 20 to 50 kilohertz, is passed through the transmitter coil. The resulting electric fields produced by this alternating current in the earth formation surrounding a well bore are detected at a spaced receiver coil by sensing the induced current or voltage in the receiver coil. Induction logging has been principally used with oil-base drilling mud or drilling fluids having high resistivities but, in recent years, has come to be used even with a highly conductive (low resistivity) drilling fluids.
In low radio frequency wave propagation, as already pointed out, the dielectric constant has practically no effect on the propagating wave. Since both the travel time and attenuation are affected in this region essentially only by the formation resistivity, measurements of these propagation parameters in low radio frequency regions yields essentially only resistivity information.
Various problems have arisen in the interpretation of either induction logging or low radio frequency wave propagation logging methods where relatively non-conductive fresh water bearing formations are encountered. Such fresh water bearing sands or formations exhibit high resistivity much the same as those encountered in hydrocarbon bearing formations. However, since hydrocarbons have a characteristically low dielectric constant and fresh water has a relatively high dielectric constant, high radio frequency propagation logs have been found to be useful in such applications. In U.S. Pat. No. 3,893,021, which is assigned to the assignee of the present invention, a solution to this problem is described utilizing a radio frequency electromagnetic field in the frequency range of 20 to 40 megahertz. At these high radio frequencies, the dielectric properties of the media surrounding the well bore influence the electromagnetic field together with the conductivity or resistivity characteristics of a material. By providing apparatus to measure both phase shift (travel time) and amplitude change (attenuation) of the signal, both the dielectric and resistivity characteristics of the earth formation in the vicinity of the borehole may be determined.
In ultra-high radio frequency logging, above 300 megahertz, wave travel time is essentially dependent only on the dielectric constant. This region is characterized by very short wave lengths and very high wave attenuation. Because of these factors, the ultra-high radio frequency logging system requires very close source to receiver spacing and hence has a very shallow depth of investigation.
Commercial DC or very low frequency AC (such as 60 Hertz) electrical resistivity logs require direct contact with the surrounding well bore by contacting electrodes. This creates problems is providing a sonde which can be easily pulled through the well bore yet make the necessary contact with the well bore. For this reason, induction logging, which does not depend on such direct contact, is more desirable. However, as earlier mentioned commercial induction electrical logging systems now available are principally used with oil-based drilling muds or drilling fluids having high resistivity and are adversely affected by brine filled or highly conductive drilling fluid filled well bores. Furthermore, commercial induction electrical logs are generally not accurate in high resistivity formations and both electrical resistivity and electrical induction logging systems commercially available have relatively poor thin-bed response; i.e. they do not give accurate resistivity values for beds thinner than four feet. It would therefore be beneficial to provide a well logging system which could provide an accurate measurement of true formation resistivity; flushed zone resistivity and invasion diameter, whether used in well bores having highly resistive drilling fluids (such as oil-base drilling muds) or higher conductive drilling fluids, and regardless of whether the formations are highly resistive or of thin beds.
Accordingly, it is an object of the present invention to provide a well logging system which is capable of accurate measurement of true formation resistivity, flushed zone resistivity and invasion diameter by means of low radio frequency electromagnetic wave propagation.
Another object of the present invention is to provide an electric resistivity well logging system which is not adversely affected by brine or other highly conductive drilling fluid filled well bores.
Another object of the present invention is to provide an electric resistivity well logging system which is accurate for both low and high resistivity formations and formations having thin beds.
Another object of the present invention is to provide a resistivity well logging system which operates at frequencies allowing a measure of true formation and flushed zone resistivities without regard for most of the problems associated with commercial dual induction resistivity logs.